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<H1>?B isd ?C</H1>
The constraint expression C evaluates to the boolean value B.


<DL>
<DT><EM>?B</EM></DT>
<DD>A variable with domain 0..1.
</DD>
<DT><EM>?C</EM></DT>
<DD>An arithmetic constraint expression.
</DD>
</DL>
<H2>Description</H2>
   This is an evaluation constraint.  It states that the constraint
   expression C evaluates to the boolean value B, where the value 0 means
   false and 1 true.  This constraint can be used both to test the validity
   of the constraint expression (entailment test) and to impose this
   constraint or its negation.  For the former, B will be instantiated as
   soon as either the constraint expression or its negation is subsumed by
   the current state of its domain variables.  For the latter, when B is
   instantiated, then depending on its value, either the constraint
   expression or its negation will be imposed on its arguments.

<P>

<H3>Fail Conditions</H3>
   Fails if both C is true.


<H3>Resatisfiable</H3>
   No.
<H2>See Also</H2>
<A HREF="../../lib/fd/HEG-3.html">#=> / 3</A>, <A HREF="../../lib/fd/HLEG-3.html">#<=> / 3</A>, <A HREF="../../lib/fd/HRF-3.html">#\/ / 3</A>, <A HREF="../../lib/fd/HFR-3.html">#/\ / 3</A>, <A HREF="../../kernel/arithmetic/is-2.html">is / 2</A>
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